3879
Inorg. Chem. 1988, 27, 3879-3880 idazole or imidazolato group from a cysteine and histidine residue, respectively, of the protein are the most favorable candidates for the axial ligands in the active site of native nitrile hydratase in the ferric low-spin state. Further characterization of the present model complexes is under way by Mossbauer, EXAFS, and X-ray structure analysis methods. (1 1) To whom correspondence should be addressed.
Faculty of Pharmaceutical Sciences University of Tokushima Sho-machi 1, Tokushima 770,Japan School of Education Yamaguchi University Yoshida 1677-1,Yamaguchi 753, Japan
Hiromu Sakurai**" Koichiro Tsuchiya Kouto Migita
Received June 3, 1988 ORTEP drawing of compound 4. Selected bond distances (A) and angles (deg): Ru(l)-O(l), 2.135 (7);Ru(l)-O(2), 2.091 (6);Ru-
Figure 1.
Siloxyl-Bridged Diruthenium(I1) Complexes: Structural Models for Cluster-Derived, Silica-Supported Ruthenium Catalysts Sir: Transition-metal catalysts dispersed on high-surface-area oxides are essential to the operation of many industrial processes, and accordingly the structure and performance of such materials is an area of continuing speculation and study. Understanding surface geometry, in particular the way in which reactive metal centers are bound to a support, is regarded as a primary objective;' however, owing to the rarity of suitable molecular models, the current state of knowledge concerning metalsupport interactions rests almost entirely on indirect characterization using various spectroscopic techniques.2 Recently the X-ray crystal structure of the triangufo-triosmium derivative [Os,(CO) &L-H) (pOSiEt,)] (1) was projected as "the first molecular analogue of the silica surface c l ~ s t e r "and ~ has since been related specifically to EXAFS analysis of alumina-supported osmium cluster^.^ We have used X-ray diffraction to determine the structures for two new siloxyl-bridged diruthenium complexes, providing reference parameters for oxide-supported ruthenium catalysts, which are numerous5 but for which no suitable molecular models have previously been described. In either compound two low-valent Ru atoms are held adjacent but are not bonded to each other (as are the Os centers3 in 1): such a configuration is likely to parallel the result of cluster fragmentation4 that accompanies catalyst activation. N M R evidence for enhanced acidity of the silanol hydrogen in the unusual complex6 2 vs that in Ru(n-cym)Cl,Ruckenstein, E.; Lee, S. H. J. Catal. 1987, 104, 259. Tohji, K.; Udagawa, Y.; Tanabe, S.; Ueno, A. J. Am. Chem. SOC.1984, 106, 612. Anderson, J. R. Structure of Metallic Catalysts; Academic: New York, 1975. Iwasawa, Y. In Tailored Metal Catalysts; Iwasawa, Y., Ed.; Reidel: Dordrecht, The Netherlands, 1986. Psaro, R.; Ugo, R. In Metal Clusters in Catalysis; Gates, B. C., Guczi, L., Knozinger, H., Eds.; Elsevier: Amsterdam, 1986. Besson, B.; Moraweck, B.; Smith, A. K.; Basset, J. M.; Psaro, R.; Fusi, A,; Ugo, R. J . Chem. Soc., Chem. Commun. 1980,569. Van? Blik, H. F. J.; Van Zon,J. B. A. D.; Huizinga, T.; Vis, J. C.; Koningsberger, D. C.; Prins, R. J. Am. Chem. SOC.1985, 107, 3139. Dobos, S.;Boszormenyi, I.; Mink, J.; Guczi, L. Znorg. Chim. Acta 1987, 134, 203. DOrnelas, L.; Choplin, A.; Basset, J. M.; Hsu, L.-Y.; Shore, S. Nouu. J. Chim. 1985, 9, 155. Duinvenvoorden, F. B. M.; Koningsberger, D. C.; Uh, Y. S.;Gates, B. C. J . Am. Chem. SOC.1986, 108,6254. For example: Zhou, X.;Gulari, E. J. Catal. 1987, 105, 499. Oukaci, R.; Sayari, A,; Goodwin, J. G. Zbid. 1987, 106, 318. Datye, A. K.; Logan, A. D.; Long, N. J. Zbid. 1988, 109, 76. Vlaic, G.; Bart, J. C. J.; Cavigliolo, W.; Furesi, A.; Ragaini, V.; Sabbadini, M. G. C.; Burattini, E. Zbid. 1987, 107,263. Jordan, D. s.;Bell, A. T. Zbid. 1987, 108,63.Darensbourg, D. J.; Ovalles, C. Znorg. Chem. 1986, 25, 1603 and references cited therein.
0020-166918811327-3879$01.50/0
(2)-O(l), 2.087 (6); Ru(2)-0(2), 2.111 (7); R~(l)-0(7), 2.085 (7); Ru(l)-O(lO), 2.183(7);Ru(2)-0(9), 2.118 (7);Ru(l)-P(2), 2.262 (3); Ru(2)-P(l), 2.321 (3);Ru(l)-O(l)-Ru(2), 100.5 (3); R~(l)-0(2)Ru(~), 101.2(3); O(l)-Ru(1)-0(2), 77.8 (2);0(1)-R~(2)-0(2), 78.4 (3); P(2)-Ru(l)-0(2), 89.3 (2);P(l)-Ru(2)-0(1), 87.3(2). The two Ru centers lie 3.247 (1) A apart.
Figure 2. ORTEP drawing of compound 6. Selected bond distances (A) and angles (deg): Ru(1)-O(l), 2.135 (10);Ru(1)-0(2), 2.086 (12);
Ru(1)-P(l), 2.325(5); Ru(l)-O(l)-Si(l), 135.4 (6);O(1)-Ru(1)-P(l), 165.1(3); P(l)-Ru(l)-C(4), 94.4(5); Ru(l)-O(l)-R ~(1)' = 103.0(4). The two Ru centers lie 3.313 (1) 8, apart.
(PPh2CH2SiMezOH)(3; cym = p-cymene; the silanol0 is not attached to Ru') suggested that treatment of 2 with H- might generate a reactive, coordinatively unsaturated siloxy-Ru species via hydrogen loss and ejection of CF3COF. During an appropriate
IsOCOCF3
PhzP-Ru-OCOCF3
oc'
I
OC 2
experiment gas evolution was indeed observed (KH, 1 mol equiv; ~~~
(6) Auburn, M.J.; Holmes-Smith, R. D.; Stobart, S. R.; Zaworotko, M. J.; Cameron, T. S.;Kumari, A. J. Chem. SOC.,Chem. Commun. 1983,
1523. (7) Brost, R.D.; Bruce, G. C.; Stobart, S.R. J . Chem. Soc., Chem Commun. 1986, 1580.
0 1988 American Chemical Society
Inorg. Chem. 1988, 27, 3880-3885
3880
that is self-evident in t h e structure of 6 under the constraints of crystallographic symmetry. The Ru2 distances of 3.247 (4) and 3.313 A (6)both lie outside bonding range; i.e. the molecules model “broken-up” Ru clusters on a silica ~ u r f a c e .Most ~ sigbinuclear complex Ru2(C0)3(0COCF,)(p-OSiMe2CH2PPh2)2nificantly, the Ru-0 distances (2.14 A, 6 mean 2.1 1 A, 4) confirm (p-OCOCF,) (4): the molecular structure is illustrated in Figure that the estimatelo of 2.17 A from EXAFS data for a decomposed 1. Simultaneously it was discovered that synthesis6 of complex ruthenium cluster on alumina is essentially correct, reinforce the 2 by action of CF3C02Hon [RU(CO),(~-S~M~~CH,PP~~)]~ (5) assertion4 that “EXAFS results are in agreement with those obis accompanied by formation of a minor constituent (
